feat(studio): group rows in the timeline, and a split disclosure

A group renders as its own row with member rows beneath it, and disclosure
splits into two independent controls: caret shows/hides a group's member
rows (structural), `∿` shows/hides any row's automation-lane rows. Plain
tracks lose their caret (nothing to disclose structurally) and keep only
`∿`. `expandedClipIds` keeps its existing keyframe-lane-state job;
`expandedGroupIds`/`expandedLaneOwnerIds` are new, independent sets.

Groups get a real position in the row/geometry pipeline rather than a
visual-only overlay: `useTimelineTrackDerivations` re-emits a group's member
tracks contiguously under a synthetic fractional anchor key
(firstMember - 0.5, the same fractional-key convention sub-composition
expansion already uses), so `rowGeometry`/keyboard-nav/virtualization treat
a group row as a first-class row without widening their key type away from
number. `TimelineLogicalRow.level` widens `1 | 2` to `1 | 2 | 3` (group /
member-under-group / lane), lanes always `owner.level + 1`.

All of it — grouped row emission, the header, the new expansion state — is
gated behind `isCanaryEnabled("audio-groups")`; disabled, `groups` resolves
empty and every new code path no-ops. `TimelineElement.audioGroup` (+
`audioGroupLabel`, resolved once per document via `resolveAudioGroups` from
B1) is parsed unconditionally, mirroring how `hidden`/`fxChain` already
flow DOM → manifest → TimelineElement — inert without the canary.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/studio/src/player/components/timelineKeyboardNavigation.test.ts

@@ -58,6 +58,10 @@ function model(overrides: Partial<Parameters<typeof buildTimelineLogicalRows>[0]
     selectedElementId: "active",
     selectedElementIds: new Set(),
     expandedClipIds: new Set(["active"]),
+    expandedGroupIds: new Set(),
+    expandedLaneOwnerIds: new Set(),
+    groups: [],
+    trackGroupOf: new Map(),
     gsapAnimations: new Map([
       [
         "active",
@@ -244,6 +248,10 @@ describe("resolveTimelineNavigationTarget", () => {
       selectedElementId: null,
       selectedElementIds: new Set(),
       expandedClipIds: new Set(),
+      expandedGroupIds: new Set(),
+      expandedLaneOwnerIds: new Set(),
+      groups: [],
+      trackGroupOf: new Map(),
       gsapAnimations: new Map(),
     });